store platform & arch information next to installed binary

#35 made clear that, in a cross-platform-packaging project using ffmpeg-static as a dependency, a second npm install should install an ffmpeg binary for a different platform/arch than before.

To enable this, install.js needs to

1. write the platform & arch next to the binary (e.g. to ffmpeg.json).
2. check if there's a ffmpeg.json in place, with a different platform/arch than desired. If platform & arch are equal, it can exit successfully.